1. Field of the Invention
The invention pertains to a hydraulic impulse screwdriver, and more particularly to a hydraulic impulse screwdriver having a compressed air motor.
2. Description of the Related Art
Impulse screwdrivers are often designed as handtools and are used extensively in different kinds of construction, in particular, for tightening screw connections. In order to assure that a screw connection will withstand the expected stresses, it is preferable that the screw connections be tightened to a specified torque. An insufficient tightening could result in a loosening of the screw connection with the associated danger of damage to the components and accidents. Tightening of the screw connection beyond the specified limiting torque will risk damaging the screw connection itself.
In order to tighten the screws or nuts to a specified torque in the sense of a controlled screw connection, it is known (DE-PS 2,600,939) how to provide a torsion element formed from a number of struts behind the driven motor shaft of the compressed-air motor and to provide a tensiometer at each of the several struts to form a part of a Wheatstone bridge. During operation of the screwdriver to tighten a screw, the reacting torque acting on the screwdriver causes a torsion of the struts about the axis of the screwdriver so that the amount of torsion is proportional to the reacting torque. This reacting torque is essentially equal and opposite to the torque that is applied to the screw. Thus, via the tensiometer and the Wheatstone bridge circuit, an electrical signal is generated that represents the instantaneous torque applied to the screw or the nut, and this electrical signal is compared with a specified limiting torque in order to ultimately turn the screwdriver off when the specified torque is reached. Except for the fact that with regard to the torsion element we are dealing with a mechanically complicated and also heavily worn component, the configuration of this torque-dependent shut-off device is difficult to access within the screwdriver and also leads to a very long design of a screwdriver, so that except for a complicated design of the screwdriver design, its handling is likewise cumbersome.
In another known torque-dependent shut-off device for an impulse screwdriver (EP-B 292,752), a torque setting device is located within the output shaft of the striker. This torque adjusting device is composed of a setting screw and a valve ball/valve seat unit under a load applied by a compression spring. With regard to the compression spring, its force acts on an adjusting rod located in the motor shaft of the compressed-air motor and the adjusting rod actuates a spring-loaded blocking valve for the compressed-air inlet to the compressed-air motor by means of a snap-in element. The shut-off device is located both at the front end of the impulse screwdriver and also behind the compressed-air motor, and as a mechanical structure is exposed to considerable wear and moreover, it is only accessible after some effort, so that this design is certainly in need of improvement.